Equaliser, or phase plug, for electro-acoustic transducers

ABSTRACT

Equaliser ( 4 ) or phase plug, for electro-acoustic transducers ( 1 ), having at least a central hole ( 40 ), and or a first series of openings ( 41 ), or slots, in the from of concentric rings, coaxial to an axis ( 4   a ) of symmetry of the phase plug ( 4 ). Said phase plug is characterised in that it has at least a second series of radial slots ( 42 ) positioned externally to said hole ( 40 ) or to said first series of slots ( 41 ). In the phase plug ( 4 ) has a third series of radial slots ( 43 ) interposed between said first and second series ( 41;42 ).

TECHNICAL FIELD AND BACKGROUND ART

The present invention relates to an equaliser, or phase plug, forelectro-acoustic transducers, having at least a central hole and/or afirst series of openings, or slots, in the form of concentric rings,coaxial to an axis of symmetry of the equaliser.

As is well known, equalisers, better known as “Phase Plug” (PP), areinserted within electro-acoustic transducers designed for soundreproduction, commonly known by the term “Compression Drivers” (CD).

A CD comprises a static magnetic circuit with axial symmetry, obtainedusing a permanent magnet, for instance with annular shape, interposedbetween two plates made of ferromagnetic material, one of which, calledcentral pole, has a holed expansion which is inserted in throughcavities of the magnet and of the other plate, which is thus annularlypositioned relative to said expansion.

Between the expansion of the central pole and the annular plate isobtained an annular air gap where the field lines of the magneticcircuit are concentrated.

The movable part of the CD is constituted by a cylindrical coil and by amembrane connected thereto.

Combining the movable part with the magnetic circuit, the coil ispositioned exactly at the centre of the air gap.

By applying a sinusoidal electrical signal to the coil, an alternatingmotion of the membrane is obtained in the direction of the axis of thestructure with the consequent creation of sound waves, which arepropagated towards the output of the CD by means of the hole present inthe expansion of the central pole.

Normally, CDs are coupled to acoustic horns which allow to amplify andfocus the sound within a limited region of space.

The PP has a hemispherical surface and is normally positioned inside thecentral cavity of the expansion of the central pole, through which soundpropagates towards the output of the CD.

The PP has a series of openings, called “slots”, obtained on thespherical surface of the PP adjacent to the membrane. The slots end onthe opposite side of the PP relative to the membrane are face the outputof the CD.

By appropriately dimensioning the slots, it is possible to increase thesound pressure level at the output of the CD and simultaneously toeliminate all interference problems, obtaining at the output of the CDwave front that is coherent throughout the operating band of thecomponent.

During the development and production stage it is extremely important toassemble the CD in such a way that the PP faithfully reproduces thespherical shape of the membrane.

A first type of PP provides for a series of slots in the form ofconcentric annular rings, symmetrical relative to the axis of symmetryof the CD.

There are various criteria for determining the number of slots, theirposition on the hemispherical surface adjacent to the membrane and theair passage area. In particular, a known criteria is Bob Smith's,subsequently amplified by Francher Murray, wherein a theoretical studyhas also been presented regarding the removal of the stationary wavesgenerated between membrane and PP.

The PP provided with slots in the form of concentric rings describedabove have some drawbacks.

First of all, said PPs are extremely costly and complicated to assemble,since they must be constructed of multiple pieces (generally, “n” if “n”is the number of slots). This makes it particularly critical to controltolerances on the distance between membrane and spherical surface of thePP.

Secondly, such an arrangement of the slots generates resonance phenomenawhich are difficult to eliminate, especially at high frequencies (over10 kHz).

A second type of PP currently on the market has slots in the form ofrectangles or trapezoids, positioned in radial direction relative to theaxis of symmetry of the transducer.

This arrangement of the slots allows to eliminate the resonance typicalof the first type of PP.

All other parameters being equal, the PP with radial symmetry allows toincrease the extension of the reproduced sound range, but at the sametime it also produces a loss of sound pressure.

Therefore, although a PP with radial symmetry has an undoubted advantagein terms of construction simplicity, it has a disadvantageous loss ofefficiency, especially at high frequencies.

There are other types of PP; they are less widely used, but they are ofsome interest nonetheless.

In particular, PPs are known which are constituted by a series ofvariously distributed circular slots, as well as PPs comprising slotsarranged in parallel, according to vertical or horizontal directions.

DISCLOSURE OF THE INVENTION

An aim of the present invention is to eliminate the aforesaid drawbacksmaking available an equaliser, or phase plug, for electro-acoustictransducers that is able to move beyond the audible band any resonancegenerated by the slots.

Another aim of the present invention is to propose a phase plug thatprevents possible phase cancellations.

A further aim of the present invention is to make available a phase plugthat eliminates high frequency resonance and simultaneously prevents thegeneration of stationary waves between PP and membrane.

Yet another aim of the present invention is to make available a phaseplug that can be made in a single piece, within the context of a simple,rational and reliable constructive solution.

Said aims are fully achieved by the phase plug of the present invention,which is characterised by the contents of the claims set out below andin particular in that it has at least a second series of radial slotspositioned externally to said hole or to said first series of slots.

BEST MODE FOR CARRYING OUT OF THE INVENTION

This and other features shall become more readily apparent from thedescription that follows of a preferred embodiment illustrated, purelyby way of non limiting example, in the accompanying drawing tables, inwhich:

FIG. 1 shows a top view of a first embodiment of a phase plug accordingto the invention

FIG. 2 shows a top view of a second embodiment of a phase plug accordingto the invention;

FIG. 3 shows a top view of a third embodiment of a phase plug accordingto the invention.

FIGS. 4 and 5 respectively show a top and a bottom enlarged view of afourth embodiment of a phase plug according to the invention;

FIG. 6 shows a section view of an electro-acoustic transducer comprisinga phase plug according to the invention.

With reference to FIG. 6, the electro-acoustic transducer is globallyindicated with the number 1 and comprises a permanent magnet 2 withannular shape and centrally provided with a through hole.

In accordance with a first embodiment variation, not shown herein, themagnet 2 may be constituted by a plurality of annular segments, each ofwhich is a permanent magnet.

In accordance with a second embodiment variation, not shown herein, themagnet 2 may be obtained by means of a plurality of permanent magnets,each having preferably circular section.

The transducer 1 comprises a first plate 3, or central pole, positionedin contact with the magnet 2 and having an expansion 3 a so shaped as tobe inserted in the through hole of the magnet itself. The expansion 3 ahas a cavity shaped complementarily to a phase plug 4, in order toreceive it and maintain it in position.

If the second embodiment variation described above is used, thepermanent magnets will be arranged peripherally to the expansion 3 a,taking care that the centre of the section of each magnet is positionedon a circumference that is substantially coaxial to the central pole 3a.

At the side opposite the first plate 3 relative to the magnet 2 ispresent a second plate 5, also positioned in contact with the permanentmagnet. The second plate 5 defines an air gap 6 in combination with theexpansion 3 a of the central pole 3.

The transducer 1 is provided with a coil 7 positioned coaxially to theexpansion 3 a of the central pole and immersed in the air gap 6. Thecoil can move with alternating motion parallel to an axis of symmetry 4a of the phase plug 4.

To the coil 7 is connected a membrane 8, typically hemispheric, whichcovers, preferably without touching it, an upper surface 4 b of thephase plug 4. The vibration of the membrane, caused by the alternatingmotion of the coil 7, generates acoustic waves which reproduce anelectrical signal transmitted to the coil by means of an electroniccircuit associated thereto.

The phase plug 4 is provided with at least a central hole 40 (see FIG.2) and/or with a first series of openings, or slots, 41 (see FIGS. 1, 2and 3), in the form of concentric rings, coaxial to the axis of symmetry4 a of the phase plug.

The phase plug is originally provided with a second series of radialslots 42 positioned externally to said hole and/or to said first seriesof slots 41.

With particular reference to the embodiment shown in FIG. 3, the phaseplug 4 has a third series of radial slots 43 interposed between saidfirst and second series 41, 42.

In the embodiments shown in FIGS. 1, 2 and 3, the second series of slots42 is obtained in correspondence with the perimeter of the phase plug.Specifically, said perimeter is defined by the lateral walls of theexpansion 3 a of the central pole. In particular, said lateral wallsserve as external closure surfaces for the slots.

With reference to FIGS. 4 and 5, the phase plug is provided with anannular central slot 44 comprising three annular segments 44 a, 44 b, 44c, necessary to provide mechanical support to the central part of thephase plug.

The phase plug is further provided with a series of radial slots 45external to the central slot 44 and preferably with trapezoidal shape.

In the embodiment described above, the open surface of the phase plug 4reserved for the passage of air is preferably about {fraction (1/10)} ofthe hemispherical surface of the membrane.

Moreover, the slots were positioned in such a way that to equal portionsof membrane correspond slots of equal dimensions.

The transducer 1 is provided with an acoustic horn 9 connected to thecentral pole 3, at the side opposite the phase plug with respect to themagnet 2. Said acoustic horn is necessary to amplify and focus the soundwithin a limited region of space.

The invention achieves important advantages.

First of all, with respect to a phase plug provided with slots in theform of concentric rings, coaxial to the axis of the phase plug, a PPaccording to the invention allows to eliminate resonance present at highfrequency and offers a more linear and more easily equalised response.

Secondly, with respect to phase plugs having exclusively radial slots, aPP according to the invention, in addition to offering a more linearresponse, also has high acoustic efficiency, especially in the frequencyband above 7 kHz.

An additional advantage is given by the fact that such a PP not onlyeliminates high frequency resonance but also prevents the generation ofstationary waves between the phase plug and the membrane.

Advantageously, since a PP in accordance with the presence invention canbe constructed in a single piece, it is economical and simple tofabricate.

Yet another advantage of the present invention is given by the fact thata PP according to the invention prevents possible phase cancellations.

1. Equaliser (4), or phase plug, for electro-acoustic transducers (1), having at least a central hole (40) and/or a first series of openings (41), or slots, in the form of concentric rings, coaxial to an axis (4 a) of symmetry of the phase plug (4), characterised in that it has at least a second series of radial slots (42) positioned externally to said hole (40) or to said first series of slots (41).
 2. Phase plug as claimed in claim 1, characterised in that it has a third series of radial slots (43) interposed between said first and second series (41;42).
 3. Phase plug as claimed in claims 1 or 2, characterised in that it is associated to an electro-acoustic transducer (1), comprising: at least a permanent magnet (2) of annular shape and centrally provided with a through hole; at least a first plate (3), or central pole, positioned in contact with the magnet (2) and having an expansion (3 a) so shaped as to be inserted in the through hole of the magnet (2) itself, said expansion (3 a) having a cavity shaped complementarily to the phase plug (4) to receive it; at least a second plate (5) in contact with the magnet (2) at the opposite side with respect to the first plate (3) and defining an air gap (6) with said expansion (3 a); at least a coil (7) coaxial to the expansion (3 a), immersed in the air gap (6) and movable parallel to an axis (4A) of symmetry of the phase plug (4); at least a movable membrane (8) positioned on an upper surface (4 b) of the phase plug (4) and connected to the coil (7).
 4. Phase plug as claimed in claims 1 or 2, characterised in that it is associated to an electro-acoustic transducer (1) comprising: at least a permanent magnet constituted by a plurality of annular segments delimiting a through hole; at least a first plate (3), or central pole, positioned in contact with the magnet and having an expansion (3 a) so shaped as to be inserted in the through hole of the magnet itself, said expansion (3 a) having a cavity shaped complementarily to the phase plug (4) to receive it; at least a second plate (5) in contact with the magnet at the opposite side with respect to the first plate (3) and defining an air gap (6) with said expansion (3 a); at least a coil (7) coaxial to the expansion (3 a), immersed in the air gap (6) and movable parallel to an axis (4) of symmetry of the phase plug (4); at least a movable membrane (8) positioned on an upper surface (4 b) of the phase plug (4) and connected to the coil (7).
 5. Phase plug as claimed in claims 1 or 2, characterised in that it is associated to an electro-acoustic transducer (1), comprising: a plurality of permanent magnets, each having circular section with the centre positioned on a circumference, said permanent magnets delimiting a through cavity; at least a first plate (3), or central pole, positioned in contact with the magnets and having an expansion (3 a) so shaped as to be inserted in the through hole delimited by the magnets, said expansion (3 a) having a housing shaped complementarily to the phase plug (4) to receive it; at least a second plate (5) in contact with the magnets at the opposite side with respect to the first plate (3) and defining an air gap (6) with said expansion (3 a); at least a coil (7) coaxial to the expansion (3 a), immersed in the air gap (6) and movable parallel to an axis (4A) of symmetry of the phase plug (4); at least a movable membrane (8) positioned on an upper surface (4 b) of the phase plug (4) and connected to the coil (7).
 6. Phase plug as claimed in claim 3, characterised in that the second series of radial slots (42) is obtained in correspondence with the perimeter of the phase plug (4), said perimeter being defined by lateral walls of the expansion (3 a) of the first plate (3), or central pole.
 7. Phase plug as claimed in claim 4, characterised in that the second series of radial slots (42) is obtained in correspondence with the perimeter of the phase plug (4), said perimeter being defined by lateral walls of the expansion (3 a) of the first plate (3), or central pole.
 8. Phase plug as claimed in claim 5, characterised in that the second series of radial slots (42) is obtained in correspondence with the perimeter of the phase plug (4), said perimeter being defined by lateral walls of the expansion (3 a) of the first plate (3), or central pole. 